In high-speed optical communication systems, optical modulators are typically used to modulate light emitted from a continuous wave (CW) laser in order to transmit voice, data, and/or video signals. Digital transmission is a special case of general signal transmission using optical modulators, most notably in that the modulation waveform is a large signal modulation, using the full intensity available from the modulator, from on to off.
Optical modulators that use a Mach-Zehnder (MZ) interferometer formed in an electro-optic substrate such as lithium niobate (LiNbO3), have been used widely in the digital transmission of optical high speed data signals, primarily due to the superior chirping characteristics, wide bandwidth, and low insertion losses thereof.
In a LiNbO3 MZ optical modulator, an RF information-bearing signal is applied to one or more of the parallel waveguides making up the interferometer arms in order to impress the information onto the light propagating therethrough. In order to achieve maximum modulation efficiency during this electrical to optical conversion, the bias (or operating) point of the MZ optical modulator is typically set at quadrature (e.g., the DC bias voltage is set to the point of inflection of the sinusoidal transfer function). Unfortunately, due to the nature of the electro-optic substrate and the interferometic principles upon which the MZ optical modulator is based, the bias point of these devices tends to drift with changes in wavelength and/or temperature, and/or as a result of aging. As a result, LiNbO3 MZ optical modulators typically require some type of bias control. Active bias control is particularly important for external optical modulators used in systems transmitting digital data at 10 Gb/s or higher, wherein bias drift can preclude good data fidelity.
Actively controlling the bias point of optical modulators, and in particular LiNbO3 MZ optical modulators, with a control loop is well known. For example, analog, digital and DSP schemes have all been used. In general, these schemes typically use some type of dither applied to the modulator, either a tone or signal summed into the signal path, or a tone or dither signal applied to the gain control of an amplifier, to produce an amplitude modulated (AM) stimulus. While dither signals typically interfere only minimally with the normal operation of optical modulators, they can unfortunately cause interference in the modulator output.
One control scheme that does not rely on a dither signal has been proposed, wherein a simple subtractive comparison of the average power of the modulator and a set-point is used. While this average power scheme works with no dither, it is, unfortunately, susceptible to errors when the average power is affected by other undesired effects.